The Study Of Dolomite Genesis Of The Middle Permian Maokou Formation In Tailai Area,Eastern Sichuan Basin

The present study focused to explain the genesis of dolomite of the Maokao Formation in the Sichuan Basin,China.The outcrop sections of Erya?Huaying Mountains?and Huilongchang?Fengdu?while drilling cores in the Tilai area are utilized for analysis.Based on in-depth outcrop measurements,observations,petrographic identification,Electron Microprobe,Scanning Electron Microscopy,XRD,Cathodoluminescence,Major&Trace element Geochemistry,Isotopes,Fluid Inclusions,and other analytic analyses;a total of four main structural components of the dolomite and various phases of dolomite fluid in the Maokao Formation have been recognized.The detailed study revealed the following points:1.The details of petrology and structural mineralogical studies show that the middle Permian Dolomites of the Maokao Formation has four main dolomite types;mosaic dolomite,tectonics controlled hydrothermal dolomite,"mist-center bright edge"dolomite,and a minor amount of cloud-formed dolomite.2.Mosaic dolomite,partially cloud-formed dolomite,and"mist-center bright edge"dolomite have formed in shallow burial diagenetic realm showing relatively oxidized conditions and marks the first phase of dolomitization in the rock unit whereby the hydrothermal fluid is transported.The geochemical analyses of Mosaic dolomite show the abundance of Ca and Mg.In contrast,the dolomites are poor in Si,Al,Mn,Sr,and Fe content.Similarly the degree of ordering ranges from 0.65 to 0.78,the Cathodoluminescence shows a dim blue light to medium orange-red,the strontium⁸⁷Sr/⁸⁶Sr isotopic values range from 0.707406 to 0.707603,and the?26Mg is-1.53%?-1.06%,all these analyses supports the syn-depositional to early diagenetic shallow burial genesis of the dolomites in relatively oxidized conditions.partially cloud-formed dolomite is in the shape of"crystal chips",The geochemical analyses of partially cloud-formed dolomite show The abundance of Sr.In contrast,The dolomites are poor In Si,Al,Mn,And Fe content.Similarly the degree of ordering ranges from 0.42 to 1.0,the Cathodoluminescence shows orange-red and has obvious ring band,it is suggested that the genesis of this kind of dolomite is not unique,it may be related to the deep-source hydrothermal solution,or it may be caused by the heterogeneous dolomization of the matrix during the burial period.3.The"mist-center bright edge"and the tectonic controlled hydrothermal dolomite mark the second phase of dolomitization and have been occurred under the conditions of shallow burial.The geochemical analysis divulged that the amount of Ca and Mg are close to or slightly higher than standard measurement.In contrast,the dolomites are poor in Si,Al,Mn,and Sr.The Cathodoluminescence of"mist-center bright edge"dolomite shows dark orange-red"bright edge"while brighter orange-red"fog center"which shows reflects two stages of formation of these dolomites.Backscattering shows that the"fog center"is more developed having more residual spots of calcite while the"bright edge"is brighter,cleaner,and denser.The degree of ordering is in between 0.54 and 0.90,which reflects the dolomite underwent a strong hydrothermal transformation and regenerated cementation based on early dolomite formation.The tectonic controlled hydrothermal dolomite is distributed in the veins,with developed ring band..The Cathodoluminescence characteristics of these dolomites show orange-yellow and dark-orange-red color.The degree of ordering is between 0.71-1.0,and the value of ⁸⁷Sr/⁸⁶Sr is higher,between0.707823-0.70859.The phenomenon of"bright edge in the center of fog"is closely related to the tectonic hydrothermal vein body.The variation range of Mg isotopic composition is basically the same,and the?26Mg is-1.53%?-1.06%,The surface bright edge development and hydrothermal vein body are formed by the action of dolomitization fluid in the same period,and the source of dolomitization fluid is of mixed origin.4.The comprehensive analysis suggests that the dolostone includes four types of microscopic fabric while dolomitization occurred in two phases:Mosaic dolomite,partially clouded dolomite,and"fog center"dolomite is the product of the first dolomitization,while"bright side"dolomite and tectonic controlled hydrothermal dolomites are formed in the second phase of dolomitization.All types of dolomites formed at the syn-depositional to shallow burial diagenetic environments.The detailed geochemical analyses of each dolomite type indicate that the source of dolomite fluid is mixed.The dolomization fluid of the mosaic dolomite is derived from the hydrothermal fluid of the Emeishan basaltic magma.However,the fluids forming the hydrothermal dolomite veins may be derived from the shallow groundwater and basaltic magma.The uniform temperature range of fluid inclusions is large,which indicates that the activity characteristics of the fluid are multi-stage and the activity intensity of the hydrothermal fluid is gradually weakened.